Word for a hollowed-out book?
The closest single-word answer for you I can find is "solander."
ODO has
A protective box made in the form of a book, for holding such items as botanical specimens, maps, papers, etc.
late 18th century: named after Daniel C. Solander (1736–82), Swedish botanist.
"Hollow book safe" and the like seem to be the only names I can find for such an object. Perhaps your friend was thinking of a steganograph, a message hidden inside a book through the use of invisible ink, a cipher, etc.